Nanofiltration (NF) membranes are often made from polymeric materials, which generally swell and dissolve, in organic solvents. The swelling of such membranes in presence of solvents and under pressure usually results in compaction, and loss of flux and performance. The potential applications of solvent stable membranes are in the areas of food technology, biotechnology, the treatment of waste streams, chemical process and petrochemical industry. One particular advantage of solvent stable membranes would be that they could be exposed to a variety of solvent media including aqueous solutions, suspensions or emulsions, as well as to organic solvents that contain solutes. These solvent stable membranes are presently available in the form of ceramics or other inorganic materials and specialized crosslinked polymers such as epoxy polyimide type polymers. These products are expensive, generally not available in the nanofiltration range and are restricted to certain configurations. Crosslinked polyimides supplied by NITTO (Japan) have some solvent resistance but are limited to the UF range.
In general, there are many references that employ polyacrylonitrile (PAN), substituted PAN or PAN co-polymers as a substrate UF membrane. In most cases, PAN is modified e.g. by crosslinking or by hydrophilization.
Many of these references also include at least one additional layer, which is often cross-linked in situ, and involve a plurality of method steps.
For example, Nawawi and Huang.sup.1, generally discloses composite chitosan based membranes made by forming a substrate membrane by casting a solution of the polymer onto a plate to form a porous membrane film, coating chitosan on the substrate membrane and cross-linking the chitosan in situ.
The use disclosed for this composite membrane is for pervaporation (PV) of feed with high isopropanol content. It is noted that pervaporation is quite different from nanofiltration. Pervaporatioin is carried out at reduced pressure, while nanofiltration is effected at positive pressure and the fluxes are orders of magnitude higher. Additionally, selective layer in a PV membrane is dense while it is porous in a NF membrane.
The reference also discloses the use of only polysulfone as the polymer for substrate membrane and hexamethylene diisocynate (HMDI) as the cross-linking agent. Notably, this cross-linker is cytotoxic. It is interesting to note that the concentrations of HMDI and glutaraldehyde required for killing 50% test animals after a 4 h inhalation period were 0.31 and 5000 ppm, respectively. It is obvious that the toxicity of HMDI is significantly greater than glutaraldehyde.
Also, it is emphasized that the concentration of chitosan used is low i.e. 0.5%/w. This results in a low viscosity of coating solution and the formation of a dense thin film. At higher concentrations of chitosan, a porous layer is formed.
Further, in Wang et al..sup.2, a composite chitosan membrane is disclosed wherein the microporous substrate membrane is PAN and the top layer is chitosan. It is noted that the PAN is hydrolyzed with NaOH. Cross-linking between the PAN and chitosan layers also includes a middle intermolecular layer.
The hydrolysis of the surface of the PAN is to facilitate the reaction of PAN and chitosan, so that tighter bonding between the two layers will occur.
The use disclosed is also for the pervaporation of alcohol.
In U.S. Pat. No. 4,985,138, which was issued on Jan. 15, 1991 to M. Pasternak, the substrate layer is PAN and the coating layer is PEI, cross-linked in situ by urea or amide linkages e.g. a polyisocyante or a poly (carbonylchloride).
Specifically, the composite membrane includes a substrate of a homo- or co-polymer of PAN, which is cross-linked, and a coating of an ionically charged hydrophilic, cross-linked polymer. An additional intermediate coating layer is also present.
Also, in U.S. Pat. Nos. 5,032,282 and 5,039,421 of Linder et al., composite membranes are disclosed which include plural coatings and method steps as well as expensive processing chemicals.